Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD AND APPARATUS FOR CREATING SUPPORT COLUMNS
USING A HOLLOW MANDREL WITH UPWARD FLOW RESTRICTORS
FIELD OF THE INVENTION
[0002] The present invention relates to the installation of aggregate piers in
foundation
soils for the support of buildings, walls, industrial facilities, and
transportation-related
structure, using displacement mandrels. In particular, the present invention
is directed
to methods and apparatus for the installation of aggregate piers through the
use of a
cylindrical hollow mandrel that includes arrangements for restricting the
upward flow
of aggregate into the mandrel during compaction.
BACKGROUND OF THE INVENTION
[0003] Heavy or settlement-sensitive facilities that are located in areas
containing soft
or weak soils are often supported on deep foundations. Such deep foundations
are
typically made from driven pilings or concrete piers installed after drilling.
The deep
foundations are designed to transfer structural loads through the soft soils
to a more
competent soil strata.
[0004] In recent years, aggregate piers have been used increasingly to support
structures located in areas containing layers of soft soils. The piers are
designed to
reinforce and strengthen the soft layers and minimize resulting settlements.
Such piers
are constructed using a variety of methods including drilling and tamping
methods such
as described in U.S. Patent Nos. 5,249,892 and 6,354,766 ("Short Aggregate
Piers"),
driven mandrel methods such as described in U.S. Patent No. 6,425,713
("Lateral
Displacement Pier"), and tamping head driven mandrel
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methods such as developed by Nathanial S. Fox and known as the "Impact Pier"
and described in
U.S. Patent No. 7,226,246.
[0005] The "Short Aggregate Pier" technique referenced above, which includes
drilling or
excavating a cavity, is an effective foundation solution, especially when
installed in cohesive
soils where the sidewall stability of the hole is easily maintained.
[0006] The "Lateral Displacement Pier" and "Impact Pier" methods were
developed for
aggregate pier installations in granular soils where the sidewall stability of
the cavity is not easily
maintained. The "Lateral Displacement Pier" is built by driving a pipe into
the ground, drilling
out the soil inside the pipe, filling the pipe with aggregate, and using the
pipe to compact the
aggregate "in thin lifts." A beveled edge is typically used at the bottom of
the pipe for
compaction.
[0007] The "Impact Pier" is an extension of the "Lateral Displacement Pier."
In this case, a
smaller diameter (8 to 16 inches) tamper head is driven into the ground. The
tamper head is
attached to a pipe, which is filled with crushed stone once the tamper head is
driven to the design
depth. The tamper head is then lifted, thereby allowing stone to remain in the
cavity, and then
the tamper head is driven back down in order to densify each lift of
aggregate. An advantage of
the Impact Pier, over the Lateral Displacement Pier, is the speed of
construction.
[0008] The invention is an improvement on such prior art techniques, and in
particular, the
Latcral Displaccment Pier, Impact Picr and their methods. A more cfficient
mcchanism is
provided for compacting aggregate by restricting upward movement of the
aggregate through the
mandrel during driving of the mandrel.
[0009] Generally, the invention employs a steel mandrel made up of an upper
pipe as a primary
portion used for the delivery of aggregate to a lower pipe portion or tamper
head. During
extraction of the mandrel, upward movement of aggregate is minimized. However,
during
compaction there is a possibility that materials may be pushed up into the
mandrel as the mandrel
is forced down. In accordance with the invention, the possibility of materials
moving up into the
mandrel is eliminated or substantially reduced.
SUMMARY OF THE INVENTION
[0010] In one aspect, the invention relates to a mandrel equipped with a flow
restrictor to avoid
aggregate moving up into the mandrel during downward compaction. The invention
is related to
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systems and methods such as described in U.S. Patent No. 6,425,713 ("Lateral
Displacement Pier") and the tamper head driven mandrel method such as
developed by
Fox and known as the "Impact Pier" and disclosed in U.S. Patent No. 7,226,246.
[0010a] In accordance with an embodiment of the present invention there is
provided a
system for constructing aggregate piers, comprising: a mandrel having an upper
portion
and a tamper head, and a passage extending therethrough for feeding aggregate
through
the mandrel to the tamper head; and the tamper head being open to provide a
passage
for aggregate to pass through the tamper head out of the mandrel, and having a
plurality
of structural members connected therein for allowing substantially unimpeded
free flow
of aggregate therethrough when the mandrel is raised during operation, and for
preventing aggregate flow back into the mandrel during downward tamping.
[0010b] Another embodiment of the present invention provides a method of
constructing aggregate piers comprising use of a mandrel having an upper
portion and a
tamper head, the upper portion and the tamper head being for allowing flow of
aggregate therethrough, the method comprising: providing a plurality of
structural
members connected inside the tamper head in a configuration for allowing
aggregate to
remain in a cavity formed by driving of the mandrel, and for allowing
substantially
unimpeded free flow of aggregate through the tamper head when the mandrel is
raised
during operation; and preventing aggregate flow back into the mandrel during
tamping
operations through engagement between the structural members and the
aggregate.
[0011] In one embodiment, the invention can employ two cylindrical pipe
portions
aligned with their adjacent ends interconnected to form an elongate mandrel. A
top
pipe portion of the mandrel is a primary aggregate delivery mechanism.
Aggregate is
fed into a hopper at the upper end of the top pipe portion. A bottom pipe
portion of the
mandrel can have a slightly larger diameter than the top pipe portion also
operates as a
tamper head for the mandrel. Structural members, which can be active
mechanical or
passive, are located within the bottom pipe portion. The structural members
allow
generally unrestricted movement of aggregate materials downward through the
mandrel
and out through the bottom pipe portion as the mandrel is lifted. When tamping
of
aggregate is conducted through the downward movement of the mandrel, the
structural
members restrict or retard the upward flow of aggregate or other materials
into the
mandrel.
[0012] In a preferred embodiment, the bottom pipe portion includes mechanical
flow
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restrictors, for example, in the form of movable vertically extending members.
The
restrictors are mounted near the top region on the interior of the bottom pipe
portion,
adjacent to the interface of the two pipe sections (although it is understood
that the top
and bottom portions could comprise a single unitary unit with varying wall
thicknesses,
etc.). The mechanical flow restrictors operate in an active and dynamic manner
to
restrict upward movement of aggregate or soil in the mandrel during tamping or
compacting operations.
[0013] In this embodiment, the mechanical flow restrictors are preferably made
up of
steel chains, wire rope, or other like mechanisms. The mechanical flow
restrictors are
typically secured at their top end inside the mandrel bottom pipe portion or
tamper
head, and extend vertically downward within the mandrel bottom pipe portion as
the
mandrel is raised. This is because the aggregate straightens out the
restrictors as the
mandrel is lifted upward. When the mandrel is moved downward during aggregate
compaction, the mechanical flow restrictors are free to move, and move inward
and
upward within the mandrel bottom pipe portion as a result of interaction with
aggregate. When the restrictors move inward, they tend to bunch up the
aggregate thus
restricting upward flow of aggregate in the mandrel.
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[0014] In a more specific embodiment, the lower end of the mandrel may also
include a
sacrificial plate (otherwise also referred to herein as a disposable driving
shoe). The sacrificial
plate is inserted into an opening at the bottom of the tamper head of the
mandrel. The plate
prevents soil from entering the mandrel during the driving operation and is
left at the bottom of
the mandrel during aggregate placement and compaction. Alternatively, the
sacrificial plate may
be eliminated and aggregate may be placed inside of the mandrel prior to
driving. The aggregate
serves to restrict soil from entering the mandrel during driving, as it is
prevented from flowing
back into the mandrel by the mechanical flow restrictors.
[0015] In constructing an aggregate pier according to the present invention,
the mandrel is driven
to its design depth. If a sacrificial plate is employed, the aggregate can be
delivered to the top of
the mandrel through the hopper that is mounted to the upper end of the
mandrel. If the mandrel
is driven without a sacrificial plate, aggregate can be fed into the mandrel
prior to driving. Upon
achieving the desired depth during the driving operation, the mandrel is then
partially extracted a
predetermined amount, e.g., typically about 3 feet, and the aggregate is
permitted to flow through
the primary mandrel delivery top portion and the larger bottom pipe portion.
The mandrel is
then driven downward, typically about 2 feet, using conventional equipment
capable of
delivering static or dynamic downward force to the bottom pipe portion of
tamper head. During
downward driving, the mechanical flow restrictors are pushed inward and upward
by the
aggrcgatc cntcring into the bottom of the mandrel. This action causes the flow
restrictors to
bunch together in the tamper head. The tamper head is then closed off in this
region by the flow
restrictors and the upward flow of aggregate in the mandrel thereby avoided or
retarded.
[0016] In an alternative embodiment, the invention is as dcscribed previously,
and also has two
cylindrical pipe portions aligned with their adjacent ends interconnected to
form an elongated
mandrel. As before, the top pipe portion of the mandrel is the primary
aggregate delivery
mechanism, and aggregate is fed into a hopper at the upper end of the top pipe
portion. The
bottom pipe portion of the mandrel has, in one embodiment, a slightly larger
diameter than the
top pipe portion, and permits unrestricted movement of the aggregate through
the mandrel when
raising the mandrel. The bottom pipe portion again serves as a tamper head for
the mandrel.
[0017] In this embodiment, passive flow restrictors are mounted on the inside
of the bottom pipe
portion, and serve to restrict upward movement of aggregate during a tamping
or compacting
operation. The passive flow restrictors are static structures and extend
generally horizontally
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inward. The passive flow restrictors may be made of steel, steel alloys, wood,
metal plates, or
other construction materials capable of providing passive resistance inside
the mandrel bottom
portion upon application of direct vertical downward movement of the mandrel.
The passive
flow restrictors are fixed along the interior periphery of the bottom pipe
portion or tamper head.
The angle of the passive flow restrictors along their top face may vary from
about 0 degrees
relative to the horizontal, to about 60 degrees downward from horizontal. They
extend into the
center of the mandrel an amount sufficient to restrict upward movement of
aggregate during
tamping, but without substantially impeding downward movement of the aggregate
relative to
the mandrel with the mandrel raised.
[0018] As with other embodiments, the lower end of the mandrel may also be
fitted with a
sacrificial plate inserted into the opening at the bottom of the tamper head
of the mandrel. In an
alternative, the plate may be eliminated and aggregate placed in the mandrel
prior to driving to
prevent soil from entering during operation. During downward driving,
aggregate entering the
bottom of the mandrel is engaged by the passive restrictors. This action
causes the aggregate
between the passive restrictors to "arch" to the restrictors, thus "clogging"
the mandrel and
preventing upward flow of the aggregate.
[0019] The present invention in all embodiments permits unrestricted gravity
flow or movement
of the aggregate relative to the mandrel while raising the mandrel and
provides for a mechanical
or passive constriction that creates a temporary aggregate plug while driving
the mandrel
downward. The aggregate plug prevents further upward movement of the aggregate
within the
mandrel and thus allows the aggregate plug to be used as an additional
compaction surface, along
with the bottom edge of the tamper head, during downward ramming. This greater
compaction
surface facilitates the construction of stronger and stiffer piers.
[0020] It is to be understood that the invention as described hereafter is not
limited to the details
of construction and arrangements of components set forth in the following
description or
illustrations in the Drawings. The invention is capable of alternative
embodiments and of being
practiced or carried out in various ways. Specifically, the dimensions as
described, and where
they appear on the Drawings are exemplary embodiments only and may be modified
by those
skilled in the art as conditions warrant.
BRIEF DESCRIPTION OF THE DRAWINGS
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[0021] Figure 1 is a front partial cross-section schematic view of a first
embodiment illustrating
a mechanically restricted mandrel in accordance with the present invention.
[0022] Figure 2 is a side partial cross-section schematic view of the mandrel
of Figure 1.
[0023] Figure 3 is a top view of the mandrel of the invention showing a hopper
for aggregate.
[0024] Figure 4 is an enlarged partial cross-section schematic view of the
bottom pipe portion or
tamper head of the mandrel of Figure 1, showing an embodiment of mechanical
flow restrictors,
for example chains, arranged around the inside periphery of the tamper head.
[0025] Figure 5 is an enlarged plan bottom view of the bottom pipe portion or
tamper head
shown in Figure 1.
[0026] Figure 6 is a perspective view of the interior of the bottom portion of
the embodiment of
Figure 1.
[0027] Figure 7 is a front partial cross-section schematic view of the mandrel
of Figure 1, as the
mandrel is being driven with a sacrificial end cap.
[0028] Figure 8 is a front partial cross-section schematic view of the
mandrel, similar to
Figure 7, as the mandrel is being extracted leaving the sacrificial end cap at
the bottom of the
cavity, and leaving a loose fill of aggregate in the cavity.
[0029] Figure 9 is a front partial cross-section schematic view of the
mandrel, similar to
Figures 7 and 8, as the mandrel is being driven downward to compact the loose
aggregate below
the bottom of the mandrel, with the flow rcstrictors deforming upwardly and
inwardly to
constrict the cross-sectional area of the tamper head, and preventing the
upward movement of the
aggregate through the mandrel by forming a temporary aggregate plug in the
bottom portion of
the mandrel.
[0030] Figure 10 is a view demonstrating arching of aggregate inside of the
bottom portion of
the mandrel to block upward flow during tamping.
[0031 ] Figure 11 is a front partial cross-section schematic view of a second
embodiment
illustrating passive flow restrictors in accordance with the present
invention.
[0032] Figure 12 is a side partial cross-section schematic view of the mandrel
shown in
Figure 11.
[0033] Figure 13 is an enlarged partial cross-section schematic front view of
the bottom pipe
portion or tamper head of the mandrel of Figure 11 with the passive flow
restrictors.
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[0034] Figure 14 is an enlarged bottom view of the bottom pipe portion or
tamper head shown in
Figure 13 showing the restrictors extending around the inner periphery of the
bottom pipe
portion
[0035] Figure 15 is a front partial cross-section schematic view of the
mandrel of Figure 11 as
the mandrel is being driven with a sacrificial end cap.
[0036] Figure 16 is a front partial cross-section schematic view of the
mandrel, similar to
Figure 15, as the mandrel is being extracted leaving the sacrificial end cap
at the bottom of the
cavity and leaving a loose fill of aggregate in the cavity.
[0037] Figure 17 is a front view of the mandrel, similar to Figures 15 and 16,
as the mandrel is
being driven downward to compact a loose fill of aggregate, with the aggregate
engaging with
the passive flow restrictors.
[0038] Figure 18 is a graph illustrating a modulus load test comparison.
DETAILED DESCRIPTION
[0039] In one aspect, a method and apparatus is provided for the installation
of aggregate piers in
foundation soils. The method consists of driving a hollow pipe mandrel I as
shown in the
Figures into the foundation soils with a base machine capable of driving the
mandrel. The base
machine is typically equipped with a vibratory piling hammer and the ability
to apply a static
force to the mandrel to achieve penctration into a foundation soil. Such
machincs are
conventional and well known in the art, and need not be described in greater
detail herein.
Alternative machines, such as those that apply dynamic force only, static
force only, or a
combination thereof may also be used.
[0040] In a preferred embodiment, as shown in Figures 1, 2, 7, 8, 9, 11, 12,
13, 15, 16 and 17,
the mandrel can have a smaller diameter top pipe portion 9 mounted on top of a
larger diameter
bottom pipe portion 2. Although the upper portion 9 and lower portion 2 of the
mandrel 1 are
shown in an exemplary manner as separate parts with the lower portion 2 of
greater outer
diameter than the upper portion9, they can take other forms. For instance, the
upper portion 9
and lower portion 2 can be made as a single integral one piece unit. Further,
the outer diameter
of the upper portion 9 can be the same as that of the lower portion 2. In such
an embodiment the
flow restrictors can be accommodated by making the wall of the lower portion 2
thinner relative
to the upper portion 9. In an exemplary embodiment, the top and bottom pipe
portions 9 and 2
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are preferably formed of standard cylindrical or articulated steel pipe having
desired size
dimensions for the aggregate pier to be constructed as will be apparent to
those of ordinary skill.
The lower end of the top pipe portion 9 is affixed to the upper end of the
bottom pipe portion 2
preferably using a ring-shaped connector plate 10 and a suitable weld or the
like, as shown in
Figures 4 and 13. The bottom pipe portion 2 serves as a tamping head. In the
embodiment of
Figures 1-10, the bottom pipe portion 2 is equipped with vertically extending
flow restrictors 6
that restrict the upward movement of aggregate through the mandrel during
compaction.
[0041] Prior to driving, the mandrel is optionally fitted with a sacrificial
plate 3 which serves as
a driving shoe and fits into an inside annulus 4 of the bottom portion 2
making up the mandrel
head. The disposable driving shoe is slightly larger than the annulus of the
mandrel head and
thus remains in position at the bottom of the mandrel 1 during driving to a
required driving
depth. When the mandrel 1 is raised, the driving shoe remains at the driven
depth and is
sacrificed as part of the operation. The sacrificial plate 3, which
constitutes the driving shoe,
may be fabricated from steel, steel alloy, wood, metal plates, or other
construction materials.
Alternatively, in place of the plate 3, the mandrel 1 may be filled with
aggregate such that when
the mandrel 1 is driven, the aggregate will form a temporary plug inside the
annular space 4.
[0042] A hopper 5 is shown throughout the Figures, in particular Figure 3, and
can be fixed (or
removably affixed) to the top of the mandrel. The hopper 5 is used to feed
aggregate into the
mandrel at any time during the operation (such as, for example, through a
slotted mandrel as described in International Patent Publication No.
WO/2006/127571.
[0043] With respect to the aggregate used with the invention, it is typically
"clean" stone with
maximum particle size of typically less than 2 inches. By the term "clean
stone" it is meant that
it typically contains less than 5% passing the No. 200 sieve size (0.074
inches). Alternative
aggregate compositions may also be used such as clean stone having maximum
particle sizes
ranging between'/4-inch and 3 inches, aggregate with more than 5% passing the
No. 200 sieve
size, recycled concrete, slag, recycled asphalt, sand, glass, and other
construction materials.
[0044] The top portion 9 of the mandrel 1 may in an alternative construction
be manufactured
using rolled steel to form a cylinder having a circular cross-section. The
bottom portion 2 of the
mandrel 1 preferably has a cross-sectional area that is slightly greater than
the cross-sectional
area of the upper portion of the mandrel. Other alternative mandrel dimensions
and shapes may
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also be used such as mandrels made from steel to fonn a square, octagonal, or
an articulated
shape.
[0045] The lower edge 8 of the bottom portion 2 of the mandrel 1 making up the
tamping head
may also be beveled outwardly, instead of straight across as shown in the
exemplary
embodiment.
[0046] The outside diameter of the top portion 9 of the mandrel I is
preferably about 10 inches
although the diameter of the top portion may vary (such as, for example, from
about 6 inches to
about 14 inches). The mandrel wall thickness may also vary, for example, from
about '/4-inch to
about one inch, depending on the mandrel diameter, length, mandrel
construction materials, and
driving conditions. The mandrel 1 is preferably about 10 to about 40 feet
long. However,
alternate lengths, for example, as short as 5 feet and as long as 70 feet may
be used. The outside
diameter of the bottom or lower pipe portion 2 is preferably about 2-6 inches
greater than the
outside diameter of the upper pipe portion 9, depending on the diameter of the
upper pipe
portion.
[0047] The bottom portion 2 of the mandrel 1 in the embodiment of Figures 1-10
contains
vertically extending moveable mechanical flow restrictors 6 affixed at their
top ends to the
undersurface of a connector plate 10 adjacent the opening at the bottom of top
pipe portion 9 as
shown in Figures 4 and 5. The flow restrictors 6 hang freely along the inside
periphery of the
bottom pipe portion 2 making up a tamper head, in a generally circular pattern
as also shown in
Figure 6.
[0048] In this embodiment, the flow restrictors 6 are preferably sixteen steel
linked chains which
form a circular array in the tamper head 2 of the mandrel 1. Depending on the
diameter of the
mandrel 1 and the tamper head, an alternate number of steel link chains may be
used in the array.
The number of links on each steel chain can also vary depending upon the size
of each individual
chain link and the height of the tamper head 2. The total length of each
individual chain is
preferably about'/a to about 2/3 of the inside height of the lower pipe
portion 2. The thickness of
each chain length varies, for example, from about'/4" to about 1". Alternative
materials, such as
wire rope or other mechanisms that resist tensile forces, but exhibit little
resistance to
compressive forces, may also be used for the upward flow restrictors 6.
[0049] In operation, the mandrel I is driven to the desired design depth. If
the sacrificial plate 3
is used, the hopper 5 is filled with aggregate after driving to the desired
design depth.
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Alternatively, the aggregate is partially or fully filled inside the mandrel
head 2 prior to driving
so that constriction of the mechanical flow restrictors 6 forms a temporary
aggregate plug in the
bottom portion 2 making up the tamper head of the mandrel 1 so that soil does
not appreciably
enter the inside of the mandrel 1 and 2 during driving to a desired design
depth.
[0050] Once the mandrel 1 reaches the design depth, it is then raised
slightly, and the sacrificial
plate 3, or the temporary aggregate plug when no plate is used, becomes
dislodged and remains
at the design depth. As the mandrel is raised, the aggregate remains in place
by moving
downward relative to the mandrel and out of the annular space 4 in the tamper
head 2. As a
result, the mandrel is raised but the aggregate remains in place, with no
appreciable additional
downward flow of aggregate. At this time, typically, the aggregate first
contacts the side wall of
the created cavity. During this operation, the mandrel I is raised, typically
about 3 feet, and then
driven back down, typically about 2 feet, to compact the aggregate that
remained as a result of
raising of the tamper head. The driving of the mandrel 1 forces the mechanical
flow restrictors 6
to constrict upward due to engaging the aggregate, thereby reducing the cross-
sectional area of
the tamper head 2. In this manner, the aggregate is prevented from flowing in
any significant
amount back up into the mandrel 1. The restriction forms a temporary aggregate
plug in the
tamper head as is illustratively shown in Figure 10.
[0051 ] In the context of the driving operation, alternative raising and
driving amounts may be
used. For example, to achieve a wider aggregate pier, the mandrel 1 may be
raised 4 or 5 feet
and then driven down 3 or 4 feet providing for a greater volume of compacted
aggregate and a
greater width of aggregate at a given depth. For applications where small
widths are desired, the
mandrel may be raised 2 feet and driven 1 foot. Other amounts can be used
depending on the
desired result as will readily be apparent to those of ordinary skill.
[0052] The temporary aggregate plug in the annular space 4 of the mandrel head
made up of the
bottom portion 2 facilitates forcing the loose lift of placed aggregate
downward and laterally into
the sidewalls of the hole and increases the pressure in the surrounding soils.
As will be readily
apparent, the pier is built incrementally in a bottom to top operation.
[0053] In an alternative embodiment as shown in Figures 11-17, the bottom
portion 2 of the
mandrel contains, for example, horizontally aligned passive flow restrictors
16 affixed about the
periphery of the bottom portion 2. In the views of Figs. 11, 12, 13, 15, 16
and 17, the flow
restrictors 16 are shown only in part at the side edges of the inner periphery
of bottom portion 2.
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In actual construction, the flow restrictors 16 typically extend around the
inner periphery of the
bottom portion 2 as more clearly shown in Figure 14.
[0054] The passive flow restrictors 16 preferably have a downwardly sloping
upper surface to
facilitate downward flow of aggregate and a horizontal or reverse sloping (not
shown) lower
surface to restrict or prevent aggregate from flowing upwardly when the
mandrel 1 moves
downwardly during compaction. The passive flow restrictors 16 extend inwardly
along the
periphery of the bottom portion 2.
[0055] As an example, in the present embodiment, three horizontal passive flow
restrictors at
different heights are shown in the bottom portion 2 and extend all the way
around the interior
circumference. The spacing between the passive flow restrictors 16 may vary,
for example, from
0.25 to 1 foot. The width of the passive flow restrictors 16 may vary
depending on the inside
diameter of the top portion 9 and bottom portion 2 of the mandrel, and on the
particle sizes of the
aggregate used. The width of the passive flow restrictors 16 is such that the
aggregate is allowed
to stay in the formed cavity (and contacting the cavity wall) by the raising
movement of the
mandrel. In contrast, passive restriction of upward flow of aggregate is
achieved during driving
of the mandrel l as a result of engagement between aggregate and restrictors
16. The number of
passive flow restrictors 16 will vary depending on the length of the bottom
portion 2. Further, as
previously noted, the flow restrictors 16 will extend into the center of the
bottom portion 2 an
amount sufficicnt to restrict upward flow of aggregate during tamping, but
without substantially
preventing the aggregate from remaining at the bottom of the cavity upon
raising of the
mandrel 1.
[0056] In all othcr aspects, the embodiment of Figures 11-17 is otherwise
typically the same as
the embodiment of Figures 1-10.
[0057] In the operation of the embodiment of Figures 11-17, as before, the
mandrel 1 is driven to
the design depth. If the sacrificial plate 3 is used, the hopper 5 is again
also filled with aggregate
after driving to the design depth. Alternatively, as in the case of the
embodiment of
Figures 1-10, the aggregate may be partially or fully filled inside the
mandrel 1 and bottom
tamper head 2 prior to driving and the aggregate is engaged by the passive
flow restrictors 16 to
form a temporary aggregate plug in the bottom portion 2 of the mandrel 1 so
that soil does not
enter the inside of the mandrel 1 during driving.
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[0058] Once the mandrel I reaches the design depth and the mandrel 1 is raised
slightly, the
sacrificial plate 3 or the temporary aggregate plug become dislodged and
remains at the design
depth. As the mandrel 1 is raised, the aggregate remains in place and moves
downward relative
to the mandrel and flows out of the annular space 4 in the lower portion 2
tamper head. In all
other aspects, the method is typically as described with reference to Figures
1-10.
[0059] In implementing the invention, it is noted that full scale installation
and field modulus
load test were performed using the embodiment of Figures 1-10 as compared to a
system such as
is described in U.S. Patent No. 7,226,246. In discussing the tests conducted,
reference is made to
Figure 18 which is a graph illustrating the results of a modulus load test
comparison between a
device such as that illustrated in Figures 1-10 as compared to a device such
as that disclosed in
U.S. Patent No. 7,226,246.
EXAMPLE
[0060] Figure 18 shows test results for two piers, one constructed using a
method similar to that
described in U.S. Patent No. 7,226,246 and one constructed using the
invention. Both piers were
built using mandrels with 14 inch diameter heads and using the 3 foot up and 2
foot down
method (as described hereinabove). The graph of Figure 18 shows that the pier
constructed with
a mandrel such as that of Figures 1-10 is stiffer than one constructed using a
system such as that
of U.S. Patent No. 7,226,246. Morc particularly, the graph shows top-of-pier
stress on the x-axis
with top-of-pier deflection on the y-axis. Volume measurements made during
construction
showed that the average pier diameter using the system in accordance with the
invention was
20% greater than that using the system of the referenced U.S. Patent.
[0061 ] In conducting the tests, the aggregate used for both systems for the
modulus load test pier
consisted of crushed limestone gravel having a nominal particle size ranging
from about 0.50 to
about 1.25 inches. The graph of Figure 18 shows a side by side comparison
where two piers
were installed to a depth of 17 to 19 feet below the ground surface. The
ground surface
consisted of fine to medium grained particle sand with little or no silt.
[0062] Modulus load tests were prepared by placing a concrete cap over the top
of the piers. The
concretc cap was installed such that a bottom of the cap was formed 24 inches
below ground
surface and the top of the cap was appropriately level with ground surface.
The cap was
24 inches in diameter such that the entire surface area of the top of the
piers were confined. The
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WO 2008/144093 PCT/US2008/054752
tests were performed by applying incremental loads to the top of the concrete
caps. A hydraulic
ram and load reaction frame was used to apply the loads.
[0063] The table of Figure 18 shows the stress at the top pier with the
deflection of the top of the
pier. The stress is determined by dividing the test load at each load
increment by the area of the
concrete cap. The deflection of the top of the pier was determined using dial
gauges on the top
of the concrete cap. The dial gauges were calibrated to have an accuracy of
0.001 inches. The
dial gauges were mounted to referenced beams that were independently supported
from the
reaction frame.
[0064] As may be appreciated from a review of the table of Figure 18, the test
results indicated
that for piers installed to similar depths and similar soil conditions using
similar aggregate
compositions, the system in accordance with the invention as illustrated in
Figures 1-10
demonstrated higher stiffness when compared to piers installed using the
system of the
aforementioned patent. This comparison was done with stiffness defined as the
stress on the top
of the pier divided by the deflection of the top of the pier at the
corresponding top of pier stress.
[0065] While the present invention has been illustrated by a description of
various embodiments
and while these embodiments have been described in considerable detail, it is
not the intention of
the Applicants' to restrict, or any way limit the scope of the appended claims
to such detail. The
invention in its broader aspects is therefore not limited to the specific
details, representative
apparatus and method, and illustrative example shown and described.
Accordingly, departures
may be made from such details without departing from the spirit or scope of
Applicants' general
inventive concept.
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